As is known, some types of electronic devices, such as Micro-Electro-Mechanical System (MEMS) devices, have black cavities where it is desired to reduce as much as possible entry and/or reflection of light.
This applies, for example, for MEMS micromirrors used in miniaturized projector modules (the so-called picoprojectors), able to project images at a distance or to generate desired patterns of light.
MEMS micromirrors generally include one or more mirror elements suspended over respective cavities and manufactured starting from a body of semiconductor material to be mobile, typically with an inclination or rotation movement, for directing the incident light beam in a desired way.
For example, FIG. 1 is a schematic representation of a picoprojector in which a light source 1, such as a laser source, generates a light beam 2 (generally formed by three monochromatic beams, one for each base color), which, through an optical system 3 represented only schematically, is deflected by a micromirror 8, here formed by a pair of mirror elements 5, 6. The first mirror element 5 may, for example, be a horizontal mirror element, which rotates about a first axis A and generates a fast horizontal scan, and the second mirror element 6 may, for example, be a vertical mirror element, which rotates about a second axis B, transverse, in particular perpendicular, to the first axis A, and generates a slow vertical scan, typically of a sawtooth type. The combination of the movements of the two MEMS mirror elements 5, 6 causes the light beam 2 to perform a complete two-dimensional scanning movement and to generate, once it has been projected onto a projection screen 7, a two-dimensional image thereon. A system of this sort is described, for example, in United States Patent Application Publication No. 2015/0217990 (corresponding to WO 2010/067354), incorporated herein by reference.
A variant of the system of FIG. 1 comprises a single mirror element of a two-dimensional type, which is rotatable both about the horizontal axis B and about the vertical axis A so as to generate the same scanning pattern as FIG. 1.
Another application of MEMS micromirrors are 3D gesture-recognition systems. These normally use a picoprojector and an image-acquisition device, such as a camera. The light beam here may be in the visible range, in the invisible range, or may have any useful frequency. The picoprojector may be similar to the picoprojector of FIG. 1, and the light beam 2 deflected by the micromirror 8 is used for scanning an object in two directions. For example, the picoprojector may project small bands on the object. Any possible projecting or recessed areas of the object (due to the depth thereof) create deformations in the rays of light detected by the camera, which may be processed by a suitable electronics for detecting the third dimension.
In both cases, with the considered technology, the rotation of the mirror element or elements is controlled via an actuation system, which is currently of an electrostatic, magnetic, or piezoelectric type.
The mirror elements 5, 6 may have the structure shown in FIG. 2 and use a piezoelectric actuation system. Here, a chip 10 of semiconductor material, such as silicon, comprises a structural layer 11 extending over a substrate 13 having a cavity 16 (see, FIG. 3 showing a corresponding cross-sectional view). The structural layer 11 forms an oscillating element 14, arranged over the cavity 16 and carrying a reflecting surface 12. Two supporting arms 15 extend on opposite sides of the oscillating element 14 and define the rotation axis of the oscillating element 14 (for example, rotation axis B of the vertical mirror element 6). The supporting arms 15 are connected via torsion springs 18 to a fixed peripheral portion 17, which is fixed with respect to the substrate 13.
The torsion springs 18 enable rotation of the supporting arms 15 about axis B as a result of actuation assemblies 20, forming a driving structure of an electrostatic type. Each actuation assembly 20 thus comprises mobile electrodes 21 facing fixed electrodes 22.
In detail, the mobile electrodes 21 are fixed with respect to the supporting arms 15 and are combfingered with respect to the fixed electrodes 22 so as to generate a capacitive coupling. The fixed electrodes 22 are carried by the fixed peripheral portion 17 fixed with respect to the substrate 13 of the chip 10. Given the arrangement of the electrodes 21, 22, the driving structure is also defined as a “comb-drive structure”.
In a known way, by applying appropriate voltages between the mobile electrodes 21 and the fixed electrodes 22, it is possible to generate attraction/repulsion forces between them so as to cause rotation of the mobile electrodes 21 with respect to the fixed electrodes 22, rotation of the supporting arms 15 about the axis B, and thus the consequent controlled rotation of the oscillating element 14. To this end, as shown in FIG. 3, contact areas 25 are formed on the fixed peripheral area 17 for electrically connecting or biasing the fixed and/or mobile regions.
In the embodiment shown, the oscillating element 14, the supporting arms 15, the torsion springs 18, the mobile and fixed electrodes 21, 22, and the fixed peripheral portion 17 are formed in the structural layer 11, and the substrate 13 closes the cavity 16 at the top.
In the mirror elements 5, 6, whether of a unidirectional type or of a two-dimensional type, spurious reflectivity, i.e., undesired reflection of light by the cavity 16, may occur. In fact, taking into account the typical dimensions of the oscillating element 14 (for example, having a diameter from 1 to 10 mm), the dimensions of the cavity 16 that enables oscillation thereof (for example, having a depth of 100-350 μm), and the tolerances of the optical assembly, part of the light directed towards the mirror elements 5, 6 may penetrate into the cavity 16. This light, delayed or modified in some of its characteristics with respect to the beam correctly reflected by the reflecting surface 12, also referred to as “stray light”, may cause an even considerable disturbance of the projection, worsening its performance.
In order to prevent or reduce spurious reflectivity as far as possible, it has already been proposed to treat the inner surface of the cavity 16 to have a roughened surface characteristic so that it behaves as a black cavity when observed under an optical microscope. In particular, it is frequently desirable to have a spurious reflectivity of less than 10%. This treatment may be carried out after etching the substrate 13 to form the cavity 16 so as to increase the surface roughness of the walls thereof.
For example, FIGS. 4-7 show successive steps of a possible process for forming and blackening (i.e., surface roughening) the cavity 16.
FIG. 4 shows a body 20 of semiconductor material, e.g. silicon, having a top surface 20A and contacts or contact pads 21, of conductive material, on the top surface 20A, for example for grounding the body 20. The body 20 (for example, a wafer designed to form the substrate 13 of FIG. 3 after dicing into single devices, in a manner known to the person skilled in the art) and the contacts or contact pads 21 are coated with an insulating layer 22, for example thermal oxide. The insulating layer 22 is removed from the top surface 20A where a cavity is to be formed. A dielectric material layer 23, for example deposited silicon oxide or nitride, extends over the top surface 20A and over the insulating layer 22, where present, for protecting the contacts or contact pads 21 during etching of the silicon and for preventing formation of cracks therein.
FIG. 5 shows the body 20 after photolithographic masking and etching the dielectric layer 23 and the body 20 for forming the cavity 16. The cavity 16 is typically formed using two separate etches to remove initially the dielectric layer 23 and then the body 20. The two etches are carried out in different apparatuses, generally as dry plasma etches, using different gases. Since the dielectric layer 23 has a protective function for the contacts or contact pads 21, the masking resist (not shown), before the etches, undergoes a curing step.
Then (FIG. 6), the cavity 16 is blackened. For example, to this end, a wet etch may be used, using an acid chemistry that includes H2SO4 at 80%, HNO3 at 10%, and HF at 10%. In this way, the side walls 16A and the bottom 16B of the cavity 16 have an irregular structure. Etching acts also laterally, forming projecting portions 24 of silicon underneath the edge of the dielectric layer 23. These projecting portions 24, also called “roofs”, may have a length of even 20-30 μm and are disadvantageous, in so far as they may have cracks that give rise to defects.
Finally (FIG. 7), the dielectric layer 23 is removed via a blanket dry etch, and the body 20 is cleaned.
Then, the following steps (not shown) are performed of bonding another body and possibly thinning it to form the structural layer 11, defining it to form the oscillating element 14 and the other structures described above, dicing the wafer, etc., in a known manner.
The described process is costly and complex, in so far as it requires numerous steps for depositing the dielectric layer 23; wet etching the walls and bottom of the cavity 16; removing the dielectric layer 23; and final cleaning, in addition to etching the silicon for forming the cavity 16. Further, it is critical because of the possible formation of cracks in the projecting portions 24, which may give rise to portions that may detach and hinder movement of the oscillating element 14.
Also known are other processes of blackening surfaces of silicon regions for different applications; for example, the paper D. Murias et al., “Black Silicon formation using dry etching for solar cells applications”, Materials Science and Engineering B 177 (2012) 1509-1513 (incorporated by reference), describes a blackening process for solar cells based upon plasma etching of the silicon surface with SF6/O2 and SF6/O2/CH4 via Reactive Ion Etching (ME). This process entails a micro-oxidation of the surface, with formation of micro-oxidized silicon that functions as micro-masking for etching silicon, and gives rise to pyramidal silicon structures. However, these structures may detach over time, and the process is thus disadvantageous in some applications, such as in micromirrors, where the presence of microparticles may jeopardize proper operation of the device.
There is a need in the art to provide a process for blackening silicon surfaces that overcomes the drawbacks of the prior art.